JP4825591B2 - Scale removal method - Google Patents

Description

  The present invention relates to a scale removal method for removing oxides adhering to a constituent member of a circulating water system.

  As is well known, power plants and substation plants are equipped with a circulating water system that circulates and cools water, and scales containing metal oxides and hardness components are deposited inside the circulating water system as the plant operates. The heat transfer effect is reduced, or the flow rate of circulating water is reduced by narrowing the flow path. Therefore, it is necessary to remove the scale. Metal oxides on the surface of these metals can be used to remove hardness components at the same time by using an inorganic acid such as sulfuric acid, hydrochloric acid or nitric acid, an organic acid such as oxalic acid or citric acid, or an aqueous solution thereof. Cleaning by is done.

  By the way, in the case of cleaning with an acid, a considerably long cleaning time is required to simultaneously remove the metal oxide and the hardness component. Therefore, a corrosion inhibitor is often added to the cleaning liquid in order to suppress and prevent the corrosion of the base material, but many of the corrosion inhibitors have a drawback of reducing the cleaning effect. In addition, various additives have been proposed to compensate for such drawbacks (Patent Documents 1 and 2).

  Patent Document 1 discloses a pickling accelerator that accelerates the oxidation rate of a metal by adding a compound represented by a specific chemical formula to an aqueous solution of an inorganic acid and / or an organic acid. Patent Document 2 discloses a copper-based metal polishing liquid containing at least one organic acid that reacts with copper hydrate to form a complex, an oxidizing agent, and water.

There is also a proposal of a cleaning liquid mainly composed of a hardness component, in which sulfamic acid is cited as one of the components of the cleaning agent (Patent Documents 3 and 4). Here, Patent Document 3 discloses a cleaning agent for cleaning a heat exchanger, piping, or the like to which a hard scale containing silicate is attached. Patent Document 4 discloses a scale remover containing sulfamic acid and organic carboxylic acid as essential components.
Furthermore, as a cleaning agent for a copper heat exchanger, a method is used in which copper oxide is dissolved with nitric acid and then copper ions are supplemented with sodium salt of EDTA (ethylenediaminetetraacetic acid).
JP-A-5-33171 JP 2003-59867 A JP 2004-277675 A JP 2000-64069 A

  However, since the object to which the present invention is applied uses pure water as circulating water, there is no or very little hardness component, and the object to be cleaned is mainly copper oxide. Moreover, since it is not high temperature corrosion like heat processing, the adhesive force of the surface copper oxide is also weak. In the case of such a cleaning target, a method of efficiently removing only copper oxide in a short time is desired. Furthermore, it is desirable to make the corrosion rate of the copper member as small as possible when the operation is resumed after cleaning.

  The conventional cleaning method is intended to remove the metal oxide and the hardness component at the same time. Therefore, while the cleaning capability is high, it is necessary to take measures to reduce the influence of the base material and other members. In addition, since the oxide film on the copper surface is removed by washing after washing, the initial copper corrosion rate after restarting operation may increase.

  The present invention has been made in view of such circumstances, without adding a component such as a corrosion inhibitor to the cleaning liquid, without damaging the base copper, and removing the scale that can suppress the copper corrosion rate after restarting operation. It aims to provide a method.

The scale removal method according to the present invention supplies an aqueous solution to which a sulfamic acid having a concentration of 0.1 to 10% by mass is added to a circulating water system provided with a constituent member to which cupric oxide is adhered, and the constituent member of the circulating water system a cleaning step of cleaning up the dissolved cupric oxide is completed, comprising a rinsing step of rinsing the circulating water system with water, and detecting by detection means of the completion of each step.

  According to the scale removal method of the present invention, since sulfamic acid is used for the cleaning liquid, the copper base material has low corrosivity, and there is no need to add a corrosion inhibitor or the like as the second component. In addition, sulfamic acid is a substance that is easier to handle than nitric acid, hydrochloric acid, sulfuric acid and the like because it is stable and odorless crystals at room temperature without any applicable laws and regulations.

  In addition, since the aqueous sulfamic acid solution selectively dissolves cupric oxide, the cuprous oxide film remains on the copper surface after cleaning, and the cupric oxide film is removed by normal acid cleaning. Corrosion rate after resumption decreases. Since cupric oxide, not cuprous oxide, adheres to the surface of the member other than the circulating water-based copper, it can be washed away with sulfamic acid.

  Further, when an aqueous sulfamic acid solution is used as a cleaning solution, the adhesion between cupric oxide and the copper base material is reduced, so that cupric oxide is easily detached with running water and then dissolved. Therefore, since the blackness of cupric oxide in the circulating water disappears, that is, by confirming the dissolution, the completion of the cleaning can be determined, so that the cleaning time can be minimized.

Hereinafter, the present invention will be described in more detail.
In the present invention, first, cleaning is performed by circulating or passing an aqueous solution added with sulfamic acid to a circulating water system including a component member to which copper oxide is adhered (cleaning step), whereby circulating water is obtained. The copper oxide on the surface of the copper member in contact is removed. At that time, only cupric oxide is selectively dissolved, and cuprous oxide remains on the copper surface. Here, the sulfamic acid causes cupric oxide to be weakened in adhesion to the copper base material and dispersed in the cleaning liquid. At that time, the cupric oxide black matter is floating in the cleaning solution, but after a while, the cupric oxide dissolves and the cleaning solution becomes transparent. This is detected by turbidity. The “turbidity” described in claim 6 means this turbidity, and when the cleaning liquid becomes transparent, it is determined that the cleaning is completed, and the cleaning completion time of the cleaning process is determined. In addition, since the oxide adhering to circulating water channels, such as a plastic piping and a ceramic resistance, is a cupric oxide, the copper oxide adhering to the surface is wash | cleaned and removed by this washing | cleaning. Therefore, the “cleaning completion time” of claim 5 can be set with a time for completing the dissolution of cupric oxide.

  In the present invention, a rinsing step is performed after the cleaning step. Here, if the cleaning solution is replaced at a time, the pH may rise rapidly and copper may precipitate, so the cleaning solution is gradually replaced with water. When the electric conductivity of the circulating water becomes the same value as the water used for the rinsing water, it is judged that the rinsing is completed. The “rinse completion time” described in claim 7 means when the conductivity of the circulating water reaches the same value as the water used for the rinse water.

In the present invention, as the sulfamic acid can be mentioned, for example, amidosulfonic acid is not limited thereto. Moreover, the density | concentration of the said sulfamic acid is 0.1-10 mass% as above-mentioned. This is because if the concentration is less than 0.1% by mass, a sufficient cleaning effect cannot be obtained. If the concentration exceeds 10% by mass, the components of the circulating water system are damaged due to a chemical reaction with the concentrated sulfamic acid. Because there is a risk of receiving.

  In the present invention, the temperature of the cleaning liquid used in the cleaning step is preferably 20 to 40 ° C. Here, if the temperature is less than 20 ° C., the cleaning effect is not sufficient, and if it exceeds 40 ° C., it is difficult to set the cleaning solution at a high temperature and the components may be damaged.

  In the present invention, the detection means for detecting the completion of each process is a measuring instrument having a function of detecting the completion of dissolution of cupric oxide in the cleaning process, or measures the turbidity or transparency of circulating water. A turbidimeter is shown. In the rinsing step, a conductivity meter for detecting the conductivity of the circulating water used for measuring the rinsing completion time is shown.

Next, specific examples will be described.
(Example)
First, a 3% aqueous solution of amidosulfonic acid (H ₂ NSO ₃ H) is prepared with pure water (distilled water, ion-exchanged water). Here, not only amide sulfonic acid but also N-alkyl derivatives of amide sulfonic acid (commonly referred to as sulfamic acid) can be substituted. If the concentration of amidosulfonic acid is 3% or more, the solubility of cupric oxide does not change. Tests at 20 ° C and 40 ° C showed little change in solubility. Table 1 below shows the results of testing the solubility of cupric oxide powder.

  Moreover, as a result of carrying out the same test as Table 1 with the sample which heat-processed the copper plate and formed the copper oxide, the surface of black cupric oxide within 10 minutes after immersion with the density | concentration of 1 wt% or more of amide sulfonic acid aqueous solution. Was removed. When 0.1 wt% and 0.5 wt% of the amide sulfonic acid aqueous solution were immersed for 30 minutes, cupric oxide was removed. At any concentration, red-brown cuprous oxide remained after removal of cupric oxide.

Table 2 below shows the results of evaluating the corrosivity (solubility of copper) with the same cleaning liquid as in Table 1 for the influence on the copper base material. From Table 2, if the temperature of the cleaning liquid is 40 ° C. or less, it can be determined that there is almost no erosion of copper within the range of the amide sulfonic acid concentration of 0.1 to 10%. Moreover, there was no difference in the test result of 20 degreeC and 40 degreeC. From the above test results, it can be seen that the amide sulfonic acid concentration has a cleaning effect even at 1 to 3 wt% from the viewpoint of the cleaning effect and the cleaning time, and the cleaning temperature is sufficient at room temperature. If the cleaning time can be secured, the concentration can be further reduced.

  Next, the removal performance of the copper oxide scale adhering to the ceramic surface and the inner surface of the Teflon (registered trademark) pipe other than copper in the circulating water system was immersed in a 3 wt% amide sulfonic acid aqueous solution to investigate the removal performance. After immersion at room temperature for 10 minutes, the black cupric oxide attached to the surface of any member was removed. Red-brown cuprous oxide as observed after washing with the heated copper plate described above was not observed after washing. In an open pure water circulation system, copper ions dissolved in water are deposited in the flow path as cupric oxide. The samples used for this test were also collected from the field. It was confirmed that the deposit was cupric oxide and could be removed with a 3 wt% amidosulfonic acid aqueous solution.

  Next, if a portion that is adversely affected by the adhesion of circulating water-type copper oxide can be obtained with the 3 wt% amide sulfonic acid aqueous solution, it is separated from the circulating water system and the cleaning liquid is circulated (cleaning step). Here, the cleaning effect is enhanced by circulation. Further, when a loop is assembled for cleaning, a cleaning liquid container is prepared, and the cleaning liquid is preferably sent from there to the loop with a submersible pump. Thereby, the progress of the removal of the copper oxide can be visually confirmed in the cleaning liquid container, which is effective in shortening the cleaning time. It can be seen that the removal of copper oxide is completed when the cleaning liquid becomes transparent. Here, the turbidimeter is set in the cleaning liquid container, and the cleaning is finished when the turbidity value is not changed.

  In the amidosulfonic acid, only cupric oxide is selectively dissolved, and cuprous oxide remains on the copper surface. Amidosulfonic acid is effective in inhibiting initial corrosion when water is circulated after restarting operation. If the cleaning solution is selected incorrectly, the oxide film on the copper surface may be completely removed after cleaning, resulting in an active surface against corrosion. In such a case, the initial copper corrosion rate may increase. This does not happen with an aqueous amide sulfonic acid solution.

  Next, rinsing with water is performed (rinsing step). If the cleaning solution is replaced at a time, the pH of the cleaning solution increases rapidly, and copper may precipitate and reattach, so the cleaning solution is gradually replaced with water. Alternatively, remove the cleaning solution from the loop and quickly circulate the water. Here, the cleaning liquid should not be left in the circulation system. The rinse does not need to be pure water, and industrial water is sufficient. When the electrical conductivity of the circulating water reaches the same value as the water used for rinsing, it is determined that rinsing is complete.

  Depending on the copper oxide deposition process, the length of the cleaning loop, and the flow rate, the scale removal process according to the present invention is often completed within 10 minutes in the cleaning process and within 3 minutes in the rinsing process.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying constituent elements without departing from the scope of the invention in the implementation stage. For example, the concentration and temperature of the cleaning liquid are not limited to the values described above, and can be set as appropriate according to the application. Further, the cleaning liquid is not limited to amide sulfonic acid, but may be other sulfamic acid. Furthermore, the setting of the cleaning completion time in the cleaning process and the setting of the rinsing completion time in the rinsing process are not limited to those described in the above embodiment. Furthermore, although the case where the amide sulfonic acid aqueous solution was circulated in the circulating water system was described in the embodiment, the present invention is not limited to this, and the aqueous solution may be supplied simply to fill the circulating water system.

Claims (5)

An aqueous solution containing sulfamic acid having a concentration of 0.1 to 10% by mass is supplied to a circulating water system provided with a constituent member to which cupric oxide is adhered, and the constituent of the circulating water system is dissolved in the cupric oxide. A scale removing method comprising: a washing step for washing until completion, and a rinsing step for rinsing a circulating water system with water, and detecting completion of each step by a detection means.   2. The scale removing method according to claim 1, wherein the sulfamic acid used in the cleaning step is amide sulfonic acid.   The scale removal method according to claim 1 or 2, wherein the temperature of the cleaning liquid used in the cleaning step is 20 to 40 ° C.   The scale removal method according to any one of claims 1 to 3, wherein the cleaning completion time in the cleaning step is determined by measuring turbidity or transparency of circulating water. 5. The scale removal according to claim 1, wherein the rinsing completion time of the rinsing process is a time when the conductivity of the circulating water becomes the same value as the water used for the rinsing water. Method.